1. Field of the Invention
The present invention relates to an image forming apparatus that forms an image using electrophotography, such as a printer, a facsimile machine or a copying machine.
2. Related Art
In general, an image forming apparatus using electrophotography includes a photoreceptor having a photosensitive layer on an outer peripheral surface thereof, charging means for uniformly charging the outer peripheral surface of the photoreceptor, exposing means for selectively exposing the outer peripheral surface which is uniformly exposed by the charging means to form an electrostatic latent, developing means for giving toners that serve as a developer to the electrostatic latent image formed by the exposing means to form a visual image (a toner image), and transfer means for transferring the toner image developed by the developing means onto a transfer medium such as a sheet.
The photoreceptors as generally known are a hard photosensitive drum on an outer peripheral surface of which a photosensitive layer is formed, and a flexible photosensitive belt on a surface of which a photosensitive layer is formed.
Also, as the charging means, the developing means and the transfer means, there have been known what are in the form of a roller which is in contact with the surface of the photoreceptor, respectively, and as its roller, there have been known a hard roller and a soft roller.
In the case where a hard photosensitive drum is used as the photoreceptor, and also a hard roller is used as the roller which is brought in contact with the hard photosensitive drum, there is naturally the limit of a high accuracy in manufacturing the photosensitive drum and the hard roller, and since an error always occurs in the accuracy, it is difficult to bring both the members in uniform contact with each other. If both the them are not in uniform contact with each other, there arises such problems that a gap is partially produced between the photosensitive drum and the developing roller to occur the ununiformity of charging, development and transfer and that these members are brought in press contact with each other stronger than a required pressure to wound the photosensitive drum or the developing roller.
From the above viewpoints, the structure in which both of the photoreceptor and the developing roller which is in contact with the photoreceptor are made of hard material is normally applied. In the case of using the hard photosensitive drum as the photoreceptor, the roller is formed of an elastic body, whereas in the case of using the hard roller as the roller, the flexible photosensitive belt is used as the photoreceptor.
However, in the case where the roller which is in contact with the photoreceptor is made of soft rubber, there arise problems stated below.
In the case where the charging roller or the like which is in contact with the photoreceptor is formed of a rubber roller, in order to give conductivity to the charging roller, conductive particles such as carbon are diffused in the charging roller. However, the hardness of the rubber is varied depending on the ununiformity or the dispersion of the degree of diffusion of carbon so that the hardness on the surface of the roller is dispersed. This causes such a problem that an excellent close contact state of the roller with the photoreceptor cannot be obtained.
On the contrary, when the amount of diffusion of carbon is reduced to obtain the excellent close contact state of the roller with the photoreceptor, there arises such a problem that the conductivity is dispersed to make charging ununiform.
Also, when, in order to enhance the flexibility, a carbon to which a plasticizer is added as a compound agent is used, there is a case in which the plasticizer is exuded from the surface of the roller. This causes a problem in which the plasticizer is stuck to the photoreceptor to change the characteristics of the photoconductive material in the photoreceptor, or a problem in which the photoreceptor is stuck to the roller to peel off the surface of the photoreceptor from the photoreceptor.
The above problems can be solved by using a hard roller as the roller and using a flexible photosensitive belt as the photoreceptor.
However, in the case of using the photosensitive drum as the photoreceptor, at least two rollers are required to support the photosensitive belt. This leads to such problems not only that the structure is complicated, but also that the device is increased in size.
In order to solve all of the above problems, up to now, there has been known a photosensitive drum disclosed in Japanese Patent Examined Publication No. Hei 4-69383 (Japanese Patent Unexamined Publication No. Sho 59-192260).
The photosensitive drum disclosed in Japanese Patent Examined Publication No. Hei 4-69383 are shown in FIGS. 1 to 3.
A photosensitive drum 1 includes a rotary shaft 2, an elastically deformable elastic material layer 3 which is supported by the rotary shaft 2 and is of a cylindrical shape in a free state, and an outer layer 4 fitted on the periphery of the elastic material layer 3. The outer layer 4 includes an elastically deformable photoreceptor support layer 5, and a photosensitive layer 6 supported by a surface of the support layer 5. The elastic material layer 3 is filled between the rotary shaft 2 and the outer layer 4 substantially without the formation of a gap therebetween.
Since the photosensitive drum 1 includes the elastically deformable outer layer 4 and the elastic material layer 3 as described above, the surface of the photosensitive drum 1 can be elastically deformed with the application of an external force to the surface.
In FIG. 1, reference numeral 7 denotes a charging charger; 10, a developing roller; and 13, a transfer charger.
In forming an image, the photosensitive drum is rotationally driven clockwise in FIG. 1, and the photosensitive layer 6 of the drum 1 is charged to a predetermined polarity by the charging charger 7. An electrostatic latent image is formed on the drum 1 by the application of a light 8 onto a charged portion. The latent image is developed by toners carried by the developing roller 10 that rotates in a direction indicated by an arrow in the figure into a visual image, and is then transferred to a transfer sheet 12 by the transfer charger 13.
It should be noted that in FIG. 1, reference numeral 14 denotes a separation charger; 15, a cleaning blade; and 16, an electrically neutralizing charger.
According to the above structure, because the surface of the photosensitive drum 1 is elastically deformable, the developing roller 10 is pushed against the photosensitive drum 1 so that the surface of the photosensitive drum 1 can be elastically deformed in a radical direction thereof. For that reason, even though the peripheral surfaces of the photosensitive drum 1 and the developing roller 10 are slightly eccentric with respect to their central axial lines, their outer diameters are slightly dispersed in a manufacturing process, or at least the surface of the developing roller 10 is formed of a rigid body, the toners on the developing roller 10 can be brought in contact with the photosensitive drum 1 in a firm and stable state in comparison with the prior art without such inconvenience that the surface of the drum or the developing roller is wounded, thereby being capable of restraining the deterioration of the quality of a visual image which is caused by the occurrence of a large gap between the toners on the developing roller 10 and the surface of the drum 1. Also, when the developing roller 10 is not abutted against the surface of the drum 1 through the toners, but the former is disposed opposite to the latter at a fine interval, the interval between the developing roller 10 and the surface of the drum 1 can be prevented from becoming excessive. This is because when the photosensitive drum 1 and the developing roller 10 are disposed close to each other, even though parts of them are in contact with each other through toners, the photosensitive drum 1 is merely elastically deformed without the photosensitive drum 1 or the developing roller 10 being wounded.
In other words, the photosensitive drum 1 thus structured can prevent the photosensitive drum or the developing roller from being wounded and also the device from being increased in size even though hard developing roller is used.
It should be noted that the photosensitive drum like the above photosensitive drum is also disclosed in Japanese Patent Unexamined Publication No. Sho 58-90655.
On the other hand, Japanese Patent Unexamined Publication No. Sho 58-86550 discloses a drum-shaped image carrier member in which, as shown in FIG. 4, an endless belt made of non-magnetic metal and 0.01 to 2 mm in thickness, which has been prepared through electroforming is used as a drum base substance 31, an image carrier layer (photoconductive material layer) 32 is formed on the drum base substance 31, and both ends of the drum base substance 31 are supported by disc-shaped end plates 33, for the purpose of lighting the device in weight and preventing an inductive eddy current from occurring.
The photosensitive drum 1 disclosed in the above-mentioned Japanese Patent Examined Publication No. Hei 4-69383(refer to FIGS. 1 to 3) is structured such that the elastic material layer 3 fills between the rotary shaft 2 and the outer layer 4 substantially without forming a gap therebetween. As a result, the photosensitive drum 1 suffers from problems stated below.
Since the photosensitive layer 6 is formed on the elastic material layer 3, the photosensitive layer 6 is deviated along the axial direction by receiving very small force. Since the developing roller 10 and the pressure member such as the cleaning blade 15, which are pressed by the photosensitive layer 6, are arranged on the photosensitive layer 6, when the rotation shaft of the photosensitive layer 6 and the shaft of the pressure member are inclined, or the pressure force becomes ununiform along the shaft direction, then the photosensitive layer 6 receives the thrust force along the axial direction, and thus the photosensitive layer 6 is deviated by this thrust force along the axial direction. Then, since this thrust force is varied, the image formed on the photosensitive layer 6 is also deviated along the axial direction. As a result, there is a problem that the positional precision of the image along the axial direction is deteriorated. In particular, when the multiple color overlapping operation is carried out, deterioration in the color overlapping precision may cause shifts in color hue. Therefore, there is another problem that the image is considerably deteriorated.
That is, as a method of manufacturing the photosensitive drum 1 thus structured, there are proposed the following three methods.
(1) A method of manufacturing the photosensitive drum 1 in which the outer layer 4 having the photosensitive layer 6 formed on the photoreceptor support layer 5 is first prepared, the shaft 2 and the outer layer 4 are then disposed at a given interval, and thereafter a heated elastic material is made to flow in a space between the shaft 2 and the outer layer 4 to form the elastic material layer 3.
(2) A method of manufacturing the photosensitive drum 1 in which the shaft 2 and the photoreceptor support layer 5 are first disposed at a given interval, a heated elastic material is made to flow in a space between the shaft 2 and the photoreceptor support layer 5 to form the elastic material layer 3, and thereafter the photosensitive layer 6 is formed on the photoreceptor support layer 5.
(3) A method of manufacturing the photosensitive drum 1 in which a cylindrical elastic member is prepared which has an outer diameter slightly larger than an inner diameter of the outer layer 4, and the cylindrical elastic member is inserted into the interior of the outer layer 4 in a state where it is compressed radially to form the elastic material layer 3.
However, the above method (1) suffers from such a problem that the photoreceptor characteristics are deteriorated by heat and so on since a work is conducted to make the heated elastic material flow into the interior of the outer layer 4 in a state where the photosensitive layer 6 is formed on the surface of the outer layer 4. Also, there is a case in which the surface of the photosensitive layer 6 is wounded, or a foreign material (a foreign material such as an elastic material) is stuck onto the surface of the photosensitive layer 6.
In the above method (2), since the photosensitive layer 6 is formed after the elastic material layer 3 is formed, the elastic material layer 3 is swelled, melted or hardened by a cleaning solvent or a coating solvent which is used during a photosensitive layer coating process, resulting in a case where the function of the elastic material layer is deteriorated.
Therefore, the above methods (1) and (2) make it very difficult to obtain a desired photosensitive drum 1.
On the other hand, in the above method (3), there is a case where during a process in which the cylindrical elastic member is expanded toward the outer layer 4 after it is released from the compressed state, the cylindrical elastic member is ununiformly expanded. As a result, the coaxial degree of the shaft 2 and the outer layer 4 becomes poor so that a shake of the photosensitive drum 1 when the photosensitive drum 1 rotates may become very large. In the image forming apparatus, because abutment members that abut against the photoreceptor, such as the charging means, the developing means, the transfer means and the cleaning means are disposed in the periphery of the photoreceptor, when the shake of the photoreceptor becomes large, a contact state of the photoreceptor with the abutment members is unstabilized, with the result that there arises a problem that an image is blurred.
On the other hand, the drum-shaped image carrier member disclosed in Japanese Patent Unexamined Publication No. Sho 58-86550(refer to FIG. 4) suffers from such a problem that because the thickness of its drum base substance 31 is thin to the degree of 0.01 to 2 mm, if an operator erroneously presses its central portion, the drum base substance 31 is damaged. Because the image carrier of this type is normally of exchangeable parts, if the image carrier may be damaged depending on its handling manner, it is extremely difficult to conduct an exchange work.
On the other hand, it is conceivable that if in the drum-shaped image carrier member (see FIG. 4) described in the above-described Japanese Laid-open Patent Application 58-86550, the drum base body 31 thereof can be easily flexed inside, this drum base body 31 may be utilized as the quasi-soft material. It can be expected to solve the problems involved in the photosensitive drum 1 (see FIG. 1 to FIG. 3) as described in the above-explained Japanese Patent Publication No. 4-69383.
However, no description about utilizations of the drum base body 31 as the quasi-soft material in this Japanese Laid-open Patent Application No. 58-86550. Moreover, conventionally, it is very difficult to fix the drum base body 31 manufactured by the electroforming method in the thinner cylindrical shape capable of readily flexing this base body inwardly by using the disk-shaped edge plate 33 in such a manner that a sectional shape thereof becomes round.
The reason is given as follows. In general, in a cylindrical body formed by way of the electroforming, thicknesses of both end portions become considerably unequal to each other, and the surfaces thereof become concave/convex. Under such conditions, first of all, a cylindrical body results whose length is longer than that of an actually used cylindrical body. Subsequently, both end portions of this cylindrical body are cut out by a cutter and the like, and thus only a central portion thereof is required to be used. Conventionally, generally speaking, when the cylindrical body is cut out, this cylindrical body is cut out while the cutter abuts against the out side of the cylindrical body. Therefore, burrs are produced on the inner circumferential side of the cylindrical body when the cylindrical body is cut out. When the cylindrical body (namely, drum base body 31) is fixed on the edge plate 33 while the burrs are being formed on the side of the inner circumferential surface of the cylindrical body, since the burrs are present between the inner circumferential surface of the base body 31 and the outer circumferential surface of the edge plate 33, it is impossible to fix the drum base body 31 on the edge plate 33 in such a manner that the sectional shape of this drum base body 31 becomes round, or circular. To achieve this fixing operation, the burrs must be removed. However, if the burrs are mechanically removed, then there are risks that the base body 31 is deformed. Also, when the burrs are removed by way of the electrolytic grinding, or polishing, the thicknesses of the both end portions becomes unequal. As a consequence, there is no meaning why the both end portions should be previously cut out.
As previously explained reasons, it is very difficult in the prior art to fix the drum base body 31 formed by way of the electroforming method by using the disk-shaped edge plate 33 in the thinner cylindrical shape capable of being easily flexed inwardly in such a manner that the sectional shape thereof becomes round.
In the case that the drum base body 31 can be readily flexed inwardly in the above-explained drum-shaped image carrier member (see FIG. 4), there is another problem that the center portion thereof is mistakenly depressed by an operator. Usually, since this sort of an image carrier is a replaceable product, when this image carrier is damaged due to a handling problem, the replacement work would become very difficult.
However, no description about utilizations of the drum base body 31 as the quasi-soft material in this Japanese Laid-open Patent Application No. 58-86550. Moreover, since this drum-shaped image carrier member (see FIG. 4) is constructed in such a way that both ends of the drum base body 31 are directly supported by the simple disk-shaped edge plate 33, the following problems will occur:
That is to say, since the fitting tolerance exists in the drum base body 31 and the disk-shaped edge plate 33, the drum base body 31 is floated from the circumferential plane of the edge plate 33 by this fitting tolerance, so that the roundness, or circularity of the drum base body 31 is deteriorated, and then the vibrations in the image carrier layer (photoconductive material layer) 32 are increased. If the fitting tolerance is reduced as much as possible, then this problem may be more or less improved. If so, then it is very difficult to fit the drum base body 31 into the edge plate 33, namely it is considerably difficult to manufacture the fitting components.
In the case that the drum base body 31 can be readily flexed inwardly, there is another problem that is the center portion thereof is mistakenly depressed by an operator, then it is destroyed. Usually, since this sort of an image carrier is a replaceable product, when this image carrier is damaged due to a handling problem, the replacement work would become very difficult.
However, since this drum-shaped image carrier (see FIG. 4) is structured by that both ends of the drum base body 31 made of Cu, Al, W, and Mo are fixed on the shaft 34 made of iron by employing the disk-shaped edge plate 33, there are the following problems.
That is to say, since the drum base body 31 is fixed on the edge plate 33 fixed on the shaft 34, and also both the shaft 34 and the drum base body 31 are constituted by the above-explained different sorts of materials, a large thermal distortion will occur inside the drum-shaped image carrier member when the temperatures are changed in the use environment and the transport environment.
As a result, the cylindrical drum base body 31 made of the thin body is distorted and deformed, resulting in vibrations. As a consequence, the intervals among the respective process members such as the developing roller are varied due to the temperature environments. Thus, it is not possible to achieve the stable/firm contact condition, or the gap management. There are risks that the image qualities are deteriorated due to fluctuations in the charging operation, the developing failure, and the transferring failure.
Also, the thinner cylindrical drum base body 31 is bent, broken, or permanently deformed due to the thermal distortion.
Furthermore, there is another risk that the fixing portions of the constructive components are destroyed due to the thermal stress.
Also, when the drum base body 31 can be readily flexed inwardly, if the central portion thereof is mistakenly depressed by the operator, then this drum base body 31 would be destroyed. Since this sort of image carrier is usually the replacement part, if this replaceable image carrier is destroyed due to poor handling, then the replacement work can be very hardly carried out.